I just ran a little test of Photoshop. In a 4k x 3k file at 32bits / channel, I created 10 layers, filled each with clouds, changed the blending modes, applied some filters, made a smart object with a smart filter on it, and did some coloring.
My memory allocation for Photoshop wound up around 5.7 GB, and I had no slowdown. I have OpenGL turned off, so there was some tearing as I panned around across the image, but I had no trouble zooming in or out.
Unfortunately, Photoshop remains the only 64-bit app that Adobe has released. I'd love to be able to use the rest of that RAM in After Effects.